1. Field of the Invention
The present invention relates to a rear projection television set in which an image light is projected from a single- or a three-tube type projection cathode ray tube onto the rear side of a screen and the light passing through the screen is viewed from the front of the screen.
2. Prior Art
In recent years, the improvement both in the brightness and in the resolution on the screen have made possible to view the rear projection television at a distance much closer to the screen than before, about five times the vertical dimension of the screen. Thus, it is required of the rear projection television set that the depth of the cabinet shown at D in FIG. 10 is as thin as possible so that the rear projection television set be space saving. For shorter depths of cabinets, the projection cathode ray tube must be located as close to the screen possible. The projector lens 2 used in the prior art rear projection television set looks substantially truly circular as depicted by S.sub.o when seen on the optical axis of lens and rather oval like a "cat's eyes" as depicted by S when seen at an angle .theta. with respect to the optical axis. This phenomenon is referred to as "Vignetting" and the degree of vignetting is referred to vignetting factor. The vignetting causes the less intensity at the peripheral portion of the screen than at middle portion. For example, in the prior art rear projection television having a 40-inch screen (600 millimeters vertically and 800 millimeters horizontally), when the distance a between the pupil of projection lens 2 and the Fresnel lens 1 is 0.8 meters, the condensing distance b.sub.1 near the center of the Fresnel lens is relatively short but the distance b.sub.2 at a full vertical dimension of the lens is about 20 meters as shown in FIG. 5. The distance b2 is referred to "vertical condensing distance" or simply "condensing distance" in this specification. When this screen is viewed at a point three meters away therefrom, about five times the vertical dimension of screen, the hot spot 5 takes up an area of about 60% of the vertical dimension of screen 4 shortly after the light exits the Fresnel lens as depicted by dotted lines in FIG. 2A, providing that no vignetting occurs. This hog spot 5 is diffused horizontally with the aid of lenticular sheet 6 so as to be viewed as a hot band or bright horizontal belt extending horizontally as depicted by dotted lines in FIG. 28 or by hatched lines in FIG. 10. Conventionally, the hot band has been eliminated by increasing the amount of diffuser in the lenticular sheet, forming the Fresnel lens 1 with a vertical lenticules 7 on the side thereof opposite to projector lens 2 as shown in FIG. 6, or providing the vertical lenticules 7 between the Fresnel lens 1 and lenticular sheet 6 as shown in FIG. 7 so as to improve diffusion of light in the vertical direction. As shown in FIG. 8, improving the vertical diffusion causes the brightness Br to vary as depicted by a curve B with the vertical viewing angle .PHI. of the screen 4 which would otherwise vary as depicted by B. The vertical angle of field is referred to a maximum vertical angle with respect to the screen where image on the screen can be viewed normally without noticeable deterioration. As is apparent from the curves A and B, the brightness of screen has been lost significantly. That is, since the heights of viewers eyes are almost always the same, increasing the vertical angle of field does not show significant improvement but the brightness of screen is rigorously deteriorated.
In order to implement thin cabinets for rear projection television sets, the light path between the cathode ray projection tube and the screen should be as short as possible. Therefore, the projection system should be of short focal lengths. In the mean time, as shown in FIG. 9, the amount E of light incident upon the screen at an incident angle of .THETA. with respect to a line normal to the screen is smaller than the amount Eo of light incident normal to the screen and is expressed as follows: ##EQU1##
If the projection system is of short focal lengths, the lights exiting the projector lenses 2 of tubes B and R differ in exiting angles .THETA..sub.1 and .THETA..sub.2 at corner portions C on the screen 4. This causes the intensity of light R to be small compared to the intensity of light B at the portions C as shown in FIG. 15. Therefore, yellow is faint near the tube R on the screen 4 while blue is faint near the tube B. Conventionally there have been two methods of improving the problem. One is to sufficiently increase the condensing distance of peripheral portions of the Fresnel lens 1, which forms the screen 4, or to cause the light to diffuse near peripheral portions of the Fresnel lens 1 so that the light from the tubes R and B are substantially the same in reflectivity. In this case, however, the amounts of light near the peripheral portions decrease when the screen is viewed at a distance of 2-5 meters away from the screen. Thus the screen is dark at its peripheral portions. Another way of solving the problem is to permit the optical axes of the tubes R and B to aim outwardly at a predetermined distance A of the center of screen as shown in FIG. 14 so that the difference of the exiting angles .THETA..sub.1 and .THETA..sub.2 is much smaller. The offset allows the exit pupil (S.sub.o in FIG. 9) of the tubes R and B to be oriented downwardly, which decreases the difference in reflectivity of the Fresnel lens 1 which in turn decreases the difference in the amount of light resulted from the aforementioned "cosine law". The second method, however, suffers from the disadvantage that doming is resulted at the hatched portions of the respective hot spots in FIG. 16 when observed from the viewer side of the screen.